A molten bath, for example a molten metal bath can have a temperature of up to 1800° C. or higher, and often needs to be monitored closely and accurately in order for many reactions or operations in the molten bath to be properly controlled. Normally, such an environment is destructive for thermocouples or other monitoring type-devices.
Radiation pyrometry, more commonly called optical pyrometry, measures the temperature of a substance by measuring the thermal radiation emitted by the substance. Thermal radiation is a universal property of matter that is present at any temperature above absolute zero. For optical pyrometry, the useful part of the thermal radiation emitted by most substances is continuous over a spectral range of approximately 0.3 to 20 μm. This spectral range encompasses the ultraviolet (UV) radiation, up to 0.38 μm; the visible (VIS) range, from 0.38 to 0.78 μm; and the infrared (IR) radiation, from 0.78 to 20 μm. IR radiation is further divided into three segments, near IR (0.78 to 3 μm), middle IR (3 to 6 μm) and far IR (above 6 μm). The distribution of the thermal radiation of a substance over the spectral range is a function of both the temperature and emissivity of the substance. Higher temperatures shift the distribution toward the shorter wavelengths. Higher emissivity increases the thermal radiation at a given temperature, whereas lower emissivity reduces the thermal radiation at the same temperature. Optical pyrometry utilises the radiating and propagating properties of matter to ascertain the temperature of a substance by measuring the intensity of the thermally radiated UV, VIS or IR energy of the substance.
In a known method, an optical monitoring device (optical pyrometer) is placed above the molten bath to measure the bath temperature. However, the temperature can be difficult to measure because an insulating slag layer is generally present over the molten bath and acts as a shield for the optical monitoring device. Further, dust can be generated in the space above the slag layer and can partially block the optical measuring device, thereby providing an inaccurate measurement of the temperature of the bath.
U.S. Pat. No. 5,302,027 discloses an apparatus for measuring the temperature of a molten bath which eliminates part of the problems discussed above. This apparatus comprises: a) a refractory mounting sleeve having an outer surface for contacting the molten bath and an inner cavity, said inner cavity having an inner surface, an outer opening and an inner closed end; and b) an optical pyrometer located on the top of the mounting sleeve and adapted to measure the thermal radiation emitted by a measurement zone located inside the inner cavity of the mounting sleeve and under the molten bath level. This apparatus is partially immersed in a molten bath. The principle of the measurement method using this apparatus is based on the fact that the thermal radiation emitted by the refractory material used as mounting sleeve is linked to the temperature of the molten bath. The refractory mounting sleeve acts as a shield that thermally protects the optical pyrometer, but also allows the measures to be made in a measurement zone located under the insulating slag layer, deep into the molten bath level.
Applicant has observed that the temperatures measured with such an apparatus were less than accurate. Therefore, a need still exists for a new apparatus for accurately and closely measuring the temperature of a molten bath.
Applicant has also established that in use, the apparatus is subject to such vibrations and shocks that the zone where the measure is read (i.e. the target or measurement zone) by the optical pyrometer moves inside the inner cavity so that accurate and reliable measures cannot be performed. Having recognised the problem, applicant has designed a new refractory sleeve adapted to receive fixedly the optical pyrometer so that this problem is overcome.